The present invention relates generally to drill bits, and, more specifically, relates to drill bits having improved cutter configurations.
Drill bits presently known to the industry which utilize stationary cutting members typically use either natural or synthetic diamonds as cutting elements, and are conventionally known as "diamond bits". References herein to "diamond bits" or "diamond drill bits" refer to all bits, for either drilling or coring, having primarily stationary cutting members. Conventional diamond drill bits may have a variety of different types of cutting surfaces, such as, for example polycrystalline diamond compact (PDC) cutters, thermally stable diamond product (TSP) cutters, mosaic-type cutters and diamond impregnated stud cutters. Mosaic cutters are typically formed of a plurality of geometrically-shaped thermally stable diamond elements cooperatively arranged and bonded in a desired shape, to form a unitary cutting surface.
On conventional bits, the diamond cutting surface is typically bonded to a supporting member, which is then secured to a carrier member which facilitates attachment to the drill bit. The cutting surface layer is preferably connected to the supporting member such as through use of brazing or sintering. Preferably, the supporting member and the carrier member will both be formed of a suitable hard or sintered metal, such as, for example, tungsten carbide. The supporting member can be soldered or otherwise connected, such as by form sintering or hot isostatic pressing, to the carrier member. A discussion of such a conventional structure is found in U.S. Pat. No. 4,498,549 issued Feb. 12, 1985 to the inventor of the present application, and assigned to the assignee of the present invention. The specification of U.S. Pat. No. 4,498,549 is hereby incorporated herein for all purposes.
Additionally, drill bits may include more than one type of cutting member on a single bit. Some types of cutting surfaces are better adapted to cut specific formations than are others. For example, while large PDC-type cutters are extremely efficient at cutting shales and other similarly soft formations, they are quickly broken or abraded in hard formations. Other cutting surfaces, however, such as mosaic surfaces, will cut through the harder formations satisfactorily. Accordingly, some relatively recent diamond drill bit designs have included dual sets of cutting members, with each set composed of cutting members having different types of cutting surfaces. With bits of that type, for example, a first set of cutters is arranged on the bit to provide essentially full face coverage across the face of the bit, and a second set of cutters, of a different type, is arranged to provide coverage across at least a portion of the bit face. An example of a bit of this type is disclosed in U.S. Pat. application Ser. No. 173,943 filed Mar. 28, 1988, in the names of Mark Jones and Mark Clench, and assigned to the assignee of the present invention.
Such bits with dual cutter sets, while performing exceptionally well in some applications, may not be feasible for use with all types of bits. For example, with bits having cutting members with large cutting surfaces, such as one inch or greater in diameter, it may not be feasible to include two sets of cutters on a single bit. Additionally, even if the additional cutter set could be included on a single bit, the extra surface area could interfere with chip removal, or could raise the total bottom hole surface area contacting the formation to a level which would require an unacceptably high weight-on-bit to cut the formation.
Accordingly, the present invention provides a new method and apparatus for including composite cutting surfaces on a single drill bit, whereby the exposed face of the cutting member may be maintained at a desired size, but whereby that face can be composed of different cutting surfaces adapted to perform differently in the earth formations.